Regulation of pancreatic acinar cell function by intracellular calcium

Enzyme secretion by the pancreatic acinar cell is regulated by the secretagogues acetylcholine (ACh) and cholecystokinin (CCK) that combine with specific membrane receptors and then induce a rise in cytoplasmic calcium. The evidence that Ca2+ mediates their actions is derived from studies demonstrating the dependence of enzyme secretion on both extracellular and intracellular Ca2+; the ability of these secretagogues to induce the release of bound or sequestered calcium; and the ability of the calcium ionophore A23187 to bypass the receptor, increase Ca2+ influx, and mimic ACh and CCK action. In contrast to many other secretory systems, however, the cellular Ca2+ that triggers secretion in response to ACh and CCK comes primarily from intracellular stores whose identity is not yet clearly established. In addition to stimulation of enzyme release, ACh and CCK have a number of other actions on pancreatic acinar cells including effects on memebrane transport and permeability, protein and lipid synthesis, and energy metabolism. At presnt, in addition to stimulating enzyme secretion, three actions of secretagogues have been clearly identified as being mediated by Ca2+: 1) stimulation of glucose transport; 2) membrane depolarization and its underlying increase in Na+ permeability; and 3) increased synthesis of cGMP. A model is presented in which intracellular Ca2+ functions as a regulatory mechanism to coordinate diverse but interrelated activities of the acinar cell.